1. Field of the Invention
This invention relates to a method and an apparatus for processing, measuring or storing high-speed optical signals.
2. Description of the Prior Art
An example of prior art optical signal processing circuit is shown in FIG. 1. In the Figure, the reference numerals 201 and 205 indicate diffraction gratings, 202 and 204 are lenses, and 203 is a spatial filter or an optical storage medium. When a time series signal light is applied to this optical circuit, a Fourier transformation of the time series signal light, that is, a frequency spectrum distribution thereof is formed on the spatial filter 203 by the frequency decomposition function of the diffraction grating 201 and the Fourier transformation function of the lens 202. When the frequency spectrum distribution is modulated by means of the spatial filter 203, the waveform of the time series signal can be modulated. Here, the waveform can be controlled by the spatial filter 203 even when the time series signal is very high in speed.
As an example, when an optical signal of a pulse width of 200 fs and a pulse interval of 5 ps shown in the upper part of FIG. 2 is applied, the incident optical spectrum has a shape as shown in the upper part of FIG. 3, having an optical power distribution shown by the broken line in the middle part of FIG. 3 on the spatial filter 203 after passing through the diffraction grating 201 and the lens 202, and when this is modulated by the spatial filter 203, it has a shape as shown in the lower part of FIG. 3 in the spectrum after passing through the spatial filter 203. A time-dependent waveform corresponding to the spectrum is the pulse sequence shown in the lower part of FIG. 3. Thus, optical signal processing can be achieved by modulating the frequency spectrum of optical signal by the spatial filter 203. That is, various waveform shaping according to the filter is possible.
Further, with 203 shown in FIG. 1 used as an optical storage medium, by applying time series signal light and reference light simultaneously, an interference fringe of both lights is hologram recorded on the optical storage medium 203. After the recording, when only the reference light is incident, the signal light is reproduced and output. Such studies are reported, for example, in A. M. Weiner, "Programable shaping of Femtosecond optical pulses by use of 128-Element Liquid Crystal Phase Modulator," IEEE J Quntun Electronics, Vol. 28 No. 4, pp. 908-920(1992); A. Weiner et al., "Spectral holography of Shaped femtosecond pulses", Optics Letters, vol. 17, pp. 224-226 (1992).
With the advance in optical communication technology, pulse widths of optical signals utilized in optical transmission are 100 ps (ex; FA-10G system) in the practical application stage, and those of next-generation very large capacity transmission apparatus are considered to utilize picosecond pulses of 1-10 ps. Optical pulses of femtosecond region are application region of the time being in the research and development and material evaluation of stable light sources, and considered not to be applied in optical communications immediately. That is, basic apparatus and method enabling optical pulse generation, waveform shaping, waveform measurement, waveform recording, correlation processing, and the like are required for constructing next-generation very high capacity systems.
However, the above-described prior art has the following problems. That is, in the modulation or hologram recording, all of the diffraction gratings 201, 203, the lenses 202 and 204, and the spatial filter 203 must be laid out in high precision, are liable to be affected by the external environment, are thus difficult to be modular structured, and are nearly impossible to operate other than in a so-called experimental environment. Therefore, it is not practicable at present stage.
Still further, when treating a time series signal, signal processing is in principle possible by single dimensional diffraction grating and lens. However, the diffraction grating and the lens have a redundant two-dimensional structure, which requires tedious positioning which is inherently unnecessary.
Yet further, when treating a long pulse sequence of over 10 ps or a pulse of large pulse width, it is required to increase the incident beam diameter which, in turn, requires large-sized diffraction grating or lens, and thus a large sized apparatus.
That is, the prior art structure using the diffraction grating pair and the lens effective for femtosecond pulses requires a very large sized apparatus for picosecond pulses, and is difficult to be packaged in a transmission apparatus of about 30.times.40.times.3 cm. Further, it is required to use a connection optical system with the optical fiber, and flexible apparatus design according to the pulses is impossible.
Heretofore, a semiconductor mode-locked laser is known as picosecond pulse generation means.
FIG. 4 shows the structure of a prior art mode-locked laser for use as a short pulse light source.
In the Figure, the mode-locked laser comprises an optical gain medium 51, an pumping circuit 52 for forming a population inversion to the optical gain medium 51, mirrors 53-1 and 53-2 constituting an optical resonator, an optical modulator 54 placed in the optical resonator, and a clock generator 55 for driving the optical modulator 54. In this construction, when the clock generator 55 drives the optical modulator 54 at a clock frequency equal to the resonance mode spacing of the optical resonator or an integer multiple thereof, an optical short pulse sequence of a repetition frequency equal to the clock frequency or an integer multiple thereof.
FIG. 5 shows the structure of a multi-wavelength light source for simultaneously oscillating light of a plurality of wavelengths.
In the Figure, the multi-wavelength light source comprises an optical gain medium 61, an arrayed-waveguide grating 62, a lens 63 for coupling the optical gain medium 61 with the arrayed-waveguide grating 62, a high reflection mirror 64 and a low reflection mirror 65 disposed at both end surfaces of the optical gain medium 61, and a high reflection mirror 66 disposed at the other end of the arrayed-waveguide grating 62.
The arrayed-waveguide grating 62 comprises an input waveguide 71, an arrayed waveguide 73 including a plurality of waveguides gradually increasing in length by a waveguide length difference .DELTA.L, a plurality of output waveguides 75, a slab waveguide 72 for connecting the input waveguide 71 and the arrayed waveguide 73, and a slab waveguide 74 for connecting the arrayed waveguide 73 and the output waveguide 75, which are formed on a substrate 70.
Light incident into the input waveguide 71 spreads by diffraction in the slab waveguide 72, and incident and distributed in equal phase into individual waveguides of the arrayed waveguide 73. The light transmitted in the individual waveguides of the arrayed waveguide 73 and reaching the slab waveguide 74 has a phase difference corresponding to the waveguide length difference .DELTA.L. Since the phase difference varies with the wavelength, when focused on the focal plane by the lens effect of the slab waveguide 74, the light is focused at different positions by wavelengths. Therefore, light of different wavelengths are taken out from the individual waveguides of the output waveguide 75.
In the multi-wavelength light source using such an arrayed waveguide grating 62, an optical resonator is formed between the high refection mirror 64 and the high reflection mirror 66, and light of a plurality of wavelengths can be simultaneously oscillated by steadily exciting the optical gain medium 61.
However, the prior art mode-locked laser has the following problems.
(1) The oscillation mode envelope spectrum is largely varied by the operation condition, and it is difficult to set the central wavelength and the pulse width.
(2) Since the amplitude and phase of each mode cannot be independently controlled, pulse shape design is difficult.
(3) A very large number of modes are excited, however, correlation between modes is insufficient due to dispersion and nonlinear effect of the semiconductor medium of the long resonator, and it is difficult to generate a transform limit optical short pulse sequence.
Further, since the phase to each mode is not controlled in the multi-wavelength as shown in FIG. 5, it is impossible to generate an optical short pulse sequence of high repetition frequency by mode locking.
As described above, there has heretofore been a semiconductor mode-locked laser as picosecond pulse generation means, however, to utilize it as a light source for optical communications, it is required that the phase and intensity are stable, the central wavelength and pulse width and pulse shape can be set (in design and fabrication), and a high quality pulse close to the transform limit is generated. However, present semiconductor mode-locked lasers are difficult to simultaneously meet these requirements. Further, it is very difficult to incorporate the prior art shown in FIG. 1 in a semiconductor mode-locked light source, and no studies have been reported on the problems.
In a very high speed optical transmission apparatus, distortion of waveform due to group velocity dispersion in the optical fiber is a first factor that limits the transmission distance. Dispersion characteristics of a transmission line (optical fiber) depend on ambient temperature, on material and cover with passage of time. Further, the dispersion characteristics are changed when the optical fiber is changed to another optical fiber in association with a malfunction or replacement of the transmission line. Or, even if the dispersion of the optical fiber is unchanged, the dispersion value applied to the optical signal is changed by changes in light source wavelength or filter characteristics.
Since even dispersion shift optical fibers of small dispersion, which are generally used, have a dispersion of about .+-.1 ps/nm/km, a transmission section of 80 km has a dispersion of .+-.80 ps/nm. Since an optical band width of optical signal of pulse width of 10 ps at 20 Gbit/s is about 1 nm, a pulse broadening of a maximum of 80 ps is generated. However, the time slot of 20 Gbit/s signal is 50 ps, a large inter-symbol interference is generated to produce large errors. Therefore, an apparatus for compensating (equalizing) the dispersion of transmission line is indispensable for a very high speed transmission apparatus.
An example of prior art is shown in FIG. 6. In FIG. 6, 01 is an optical amplifier, 02 is an optical switch, and 03 is a dispersion compensation fiber.
In the prior art, the optical signal is passed through another optical fiber having dispersion characteristics reverse to the dispersion of the transmission line to compensate the dispersion, thereby obtaining a good waveform.
Since dispersion characteristics of the dispersion compensation fiber 03 are not variable, it is general to provide fibers of different dispersion characteristics for compensating the dispersion according to changes in dispersion characteristics of the transmission line.
However, the prior art has the following problems.
(i) Compensation is difficult for high order dispersion.
(ii) A number of fibers must be provided to compensate the dispersion. In particular, since the tolerance of dispersion is narrow in a very high speed optical signal, fibers of small increment in dispersion value are required. As a result, the apparatus becomes large in size, and an optical switch of multiple switches is required.
(iii) Since the dispersion compensation fiber is switched by an optical switch, a momentary disconnection of optical signal occurs during switching.
Further, as other prior art examples, there are constructions of chirped fiber grating and multiple-connected MZ interferometer, however, these have the following problems.
(iv) Control width of central wavelength of dispersion compensation is small, and the compensation band width is narrow.
In a very high speed optical transmission apparatus, a second factor of limiting the transmission distance is distortion of waveform due to self-phase modulation in the optical fiber. Another example of prior art is shown in FIG. 7. In FIG. 7, the symbol 04 indicates an optical transmitter, 05 is a high dispersion fiber, 06 is an optical amplifier, 07 is an optical transmission line, 08 is a dispersion compensation circuit, and 09 is an optical receiver.
This construction provides a transmission method in which an optical signal is previously passed through a high dispersion medium on the assumption of compensation to increase the pulse width, thereby reducing the self-phase modulation. Since the self-phase modulation is generated nearly proportional to the optical pulse peak intensity, the modulation can be reduced by increasing the pulse width to decrease the peak power. Since, dispersion can be compensated to reproduce the waveform, however, degradation of waveform due to self-phase modulation, which is a nonlinear phenomenon, cannot be reproduced by a normal linear waveform equalization method, a transmission apparatus is required which does not generate a nonlinear phenomenon in the transmission line.
However, the prior art has the following problems.
(v) Since changes in dispersion in the transmission line cannot be previously estimated, there is a possibility that dispersion is equalized by chance in the course of the transmission line, resulting in self-phase modulation leading to an increase in waveform degradation and errors.
As described above, as picosecond pulse waveform shaping (e.g., dispersion compensation) means, there are other optical fibers having reverse dispersion characteristics to the dispersion in the transmission line, a chirped fiber grating, and a multiple-connected MZ interferometer. However, these means are difficult to achieve compensation for high order dispersion, variable dispersion compensation, and wide band compensation. Further, the prior art shown in FIG. 1 is very small in compensatable dispersion amount. For example, for an optical pulse of 2 ps in pulse width, it can provide only a compensation of about 5 ps/nm.
The optical signal processing apparatus as described above is required to measure waveform of optical signals of higher speed.
Heretofore, as waveform measuring and recording means, there is a very high speed O/E converter or a streak camera. However, the band width of the O/E converter is as much as 50 GHz and impossible to measure picosecond pulses of 1-10 ps. Further, since the streak camera is low in sensitivity in the optical communication wavelength region, a sufficient S/N is not obtained by a single sweeping, and a real-time waveform cannot be observed. There is no report of study using the prior art shown in FIG. 1. When the prior art is applied, as is, since light is distributed two-dimensionally on the Fourier transformation plane, measurement of high S/N is difficult unless a specific optical system is devised.